A clock signal is a very important signal in a communication system. The generation and transmission of various information nearly all depend on the clock, and the clock performance directly influences the performance of the entire communication system. Frequency deviation is one of the most common and important indicators of the clock performance.
The frequency deviation of a clock is generally described in two ways: absolute frequency deviation and relative frequency deviation. The absolute frequency deviation is the absolute value of a clock frequency deviation. For example, with respect to a standard frequency of 10 MHz, the absolute frequency deviation of a clock of 10.1 MHz is 100 KHz. The relative frequency deviation is a deviation percent of a clock frequency relative to the standard frequency. For example, with respect to a standard frequency of 10 MHz, the relative frequency deviation of a clock of 10.1 MHz is 1%. With respect to a certain fixed standard frequency, the absolute frequency deviation and the relative frequency deviation can be directly converted to each other.
An existing method for detecting frequency deviation uses a high frequency (HF) clock (hereinafter called as “counting clock”) to count a low frequency (LF) clock to be detected, so as to determine how many cycles of the counting clock are there in one or more cycles of the LF clock to be detected, as shown in FIG. 1.
If the counting clock is the clock having a standard frequency fn, and the counting result of counting the LF clock to be detected by the counting clock in one cycle is N, the actual frequency of the LF clock to be detected is
            f      n        N    ,and the deviation of the LF clock from the standard frequency
            f      x        ⁢                  ⁢    is    ⁢                  ⁢                  (                                            f              n                        N                    -                      f            x                          )            /              f        x              =                    f        n                    N        ·                  f          x                      -    1.  
In addition, when there is no HF clock that has a standard frequency, an LF clock having a standard frequency and the above mentioned counting clock may be adopted to count the clock to be detected, so as to acquire two counting results for comparison, and acquire the deviation of the clock to be detected, as shown in FIG. 2.
Provided that the frequency of the LF clock having a standard frequency is the expected standard frequency of the clock to be detected, the counting clock is adopted to count the clock having the standard frequency in one cycle, and the counting result is M, then the frequency deviation of the clock to be detected is
            (                                    f            n                    M                -                              f            n                    N                    )        /                  f        n            N        =                    N        ⁡                  (                      N            -            M                    )                    MN        =                  N        M            -      1.      
The shortage of the existing technology is that in order to achieve a sufficient detection accuracy of frequency deviation, the multiple between the frequency of the clock to be detected and the frequency of the counting clock must be enough. For example, when the detection accuracy of frequency deviation is expected to be 1%, the frequency of the counting clock must be more than 100 times of the frequency of the clock to be detected; and when the accuracy is expected to be 0.01%, the multiple between the two frequencies must be more than 10,000. Thus the frequency of the counting clock shall be increased, or the frequency of the clock to be detected shall be decreased, while the former sometimes cannot be implemented during actual applications, and the latter brings a negative influence that the time for detection is increased.